Large screen images, particularly moving ones, have been captivating people's imagination ever since the Lumiere brothers showed their ‘cinema’ images in 1895. For almost the next 100 years, the predominant technology for large screen moving images was using projected motion picture film. Because of the costs involved in producing Cinema film prints for distribution, together with the fact that motion picture film is easily damaged, other technologies were investigated in which the film or picture and sound information was stored digitally. This involved the development of electronic projection technologies. These technologies consisted primarily of LCD (Liquid Crystal Display) techniques and DMD (Digital Micro-mirror Device) Techniques. It is in relation to this second technology that we have made significant improvements.
Digital Micro-mirror Devices (DMDs) are a light valve first proposed by Dr. Larry Hornbeck and Dr. William E. Nelson of Texas Instruments (TI) in 1987.
A DMD is provided with several hundred thousand or indeed several million hingedly mounted microscopic mirrors. The mirrors are arranged in an array and each mirror corresponds to a pixel in the display image. The mirrors can be tilted independently towards the light source in a projection system (ON) or away from the light source (OFF) to create a light or dark pixel on the projection surface.
To display a grey scale pixel, the mirrors may be toggled on and off quickly, typically several thousand times each second. A light grey pixel is displayed if the mirror is on more than it is off; and, conversely, a dark grey pixel is displayed if the mirror is off more than it is on. A contemporary DMD may produce up to 1024 shades of gray.
A single-chip DMD may be used in conjunction with a colour filter wheel to enable colour images to be projected. Alternatively, multiple DMDs may be provided, each DMD handling a separate colour. A professional system may contain three DMDs each dedicated to handling a separate colour (red, green or blue).
Projection systems based on DLP technology were first demonstrated in 1993 and have been commercially available since around 1996, and typical models of projector are those designed and manufactured by the applicant, such as the ‘Lightning Pro’ series of projectors.
For consumer purposes, the manufacturers, Texas Instruments Inc, of Dallas, Tex., USA, have produced chips which conform to the likely domestic standard of 1080p (indicating a vertical resolution of 1080 lines, progressively displayed, and of 1920 elements per line). Since these chips are intended for domestic units, which are more price sensitive, the overall size of the DMD has been reduced. Providing more chips on each silicon wafer allows the cost of each chip to be reduced. The size reduction is typically achieved by reducing the mirror size down from a nominal 17 micrometer pitch to 10.8 micrometers. The brightness of image, which is important to potential consumers of such systems, is primarily limited by the illumination of the DMD. Typical brightness desired in domestic systems are a few thousand lumens, but professional systems can exceed 20,000 ANSI Lumens. To achieve an equal brightness of previous 17 micrometers pitch systems with the 10.8 micrometer devices, it is necessary to concentrate the same amount of light onto a smaller area. This in turn means more heat per unit area is applied to the DMD device.
There are strict limits to the maximum temperature of the DMD and also limits on the temperature gradients across the DMD. Various techniques have been developed for cooling silicon chips, in the form of microprocessors, memories, and signal processing devices, see, for example, Colgan (U.S. Pat. No. 6,774,482), Park (U.S. Pat. No. 6,687,125) and Borkar (U.S. Pat. No. 5,978,228). However, these technologies are generally not capable of working with optical semiconductors onto which it is necessary to shine high intensity light. It is assumed in the three cited US specifications above that the front, back, and sides of the device can be covered with heat sinks or similar devices. This is not practicable for DMDs since the mirrors would be obscured.
The inventors in the present case have recognized the need to provide improved cooling for DMDs, without obscuring the face through which light is applied to the mirrors to enable the DMDs to be used for high brightness projectors.
The present invention, at least in preferred embodiments, relates to cooling processes and apparatus suitable for maintaining DMDs within recommended temperature parameters, whilst still achieving high brightness projected images.